Here are a few thoughts Gallowglacht...
Helmets are feature in the DD rules because they feature in the original rules. You should house rule them if that pleases you! But be sure to notice the helmet rules given under magical helms
. One of my own house rules is to extend the rule given explicitly for magical helms to all helms. A damage die of 6 would then indicate a head hit for 6 hit points of damage. With no helmet, that is gonna hurt. With a helmet, the helmet is struck off instead causing no damage, but possibly destroying the helm.
Regarding small/large weapons:
It's really up to you. As you're deciding what to do remember that a combat turn is one minute long and subsumes multiple blows into a single attack roll. So while larger weapons might "hit harder", smaller weapons might hit more frequently, so the net effect might be about the same. Remember also that weapons are already differentiated by the order of attacks
rules, and (most significantly) by their frequencies on the treasure
Regarding first adventures:
There are so many good ones to choose from, but possibly one of the greatest introductory adventures from TSR is The Keep on the Borderlands
by EGG. It's written for Basic D&D, but it works perfectly fine with OD&D/DD too.
As for OSR dungeons, there's a lot of great and varied material in the original One Page Dungeon Codex
which is in fact system neutral, but again works perfectly fine with OD&D/DD too. As it happens I'm in the process of rewriting my own entry in that Codex (the winning entry for that contest in fact!) expressly for DD. Coming soon...
edit: Ah, I see you've got a bunch more tips over here
Hope that helps some Gallowglacht
the Elf Medium (MV 12", AC 9, HD 1, hp 1/1, AL N) great cloak, lantern
; spells: color spray; scrolls: sleep, sleep, charm person
Hirelings: Georges; torch